A fuel cell is a power generation system which oxidizes a fuel through an electrochemical process and directly converts an energy released as a result of the oxidation reaction to an electric energy. The fuel cell has a stack structure which has a plurality of membrane-electrode assemblies stacked therein, the membrane-electrode assemblies each being configured from: an electrolyte membrane which selectively transports hydrogen ions; and a pair of electrodes which are made of porous materials and which sandwich both surfaces of the electrolyte membrane. Among fuel cells of this type, it has been expected, in particular, that a solid polymer electrolyte fuel cell which uses a solid polymer membrane as an electrolyte will be used as an in-vehicle power source due to its low cost, ease of downsizing, and high power density.
In general, the optimum temperature range for a fuel cell of this type to generate electric power is considered to be 70 to 80° C. However, in cold regions or similar environments, where it often takes a long time to reach the optimum temperature range after starting the fuel cell, various types of warm-up systems have been studied. For example, patent document 1 below discloses a technique for warming up a fuel cell while allowing a vehicle to travel by carrying out a low-efficiency operation with a lower power generation efficiency as compared to a normal operation and thereby controlling the amount of self-heat generation of the fuel cell. In such a technique, since the output voltage of the fuel cell is set to a voltage value lower than a voltage value obtained based on its current-voltage characteristics (hereinafter referred to as the “I-V characteristics”) to increase heat loss of the fuel cell and a warm-up operation is carried out through self-heat generation, the technique does not need installation of a warm-up system and thus is very convenient.
Patent document 1: Japanese laid-open patent publication No. 2002-313388